In conventional pattern tracing systems of the optical type, a scanner mirror forms part of an optical system which normally projects an image of a photocell onto a line or an edge which is part of the pattern to be traced. When the scanned image of the photocell traverses the pattern, signals are obtained from the photocell which are used to derive directional and displacement signals which in turn are used to derive signals for X and Y coordinate servo-systems that maintain the optical scanning system positioned properly above the pattern while driving it along the pattern at a preselected speed.
The speed signals for the tracing head are generally set to a predetermined value on a control panel, which value when multiplied by the sine and cosine of the angle that the pattern makes with a reference direction form individual coordinate speed signals which are usually directly added to servo-input signals. Sine and cosine signals are derived from the optical scanning information, either with electromechanical synchro-resolvers in the tracing head or with electronic sine/cosine resolving circuits, and these signals are fed to the respective servo-motors to control servo-motor speed along each coordinate so that the tracing head follows the pattern to be traced.
Optical pattern tracers of this type are widely used in the industry to guide machine tools such as flame cutting machines or milling machines along a path identical to that on a flat or two-dimensional pattern. Examples of these systems are illustrated in the Barry et al. U.S. Pat. No. 2,499,178, the Brouwer U.S. Pat. No. 3,017,552, and the Jewel U.S. Pat. No. 3,322,952.
In optical line tracing systems, pattern-line catching has always been a difficult problem and in fact many commercially available tracers have totally unreliable line-pattern catching systems. Still other pattern catching systems known in the prior art are capable of achieving adequate pattern catching only when the pattern is approached at a shallow angle by the tracing head. Frequently the pattern catching systems in known optical pattern tracers either fail to catch the line at all or are uncertain as to the direction of travel following catching. Furthermore, pattern catching systems presently known are incapable of distinguishing direction once the scanner is locked onto a line.
The present invention relates particularly to optical scanners, that project a photocell in a circular or eliptical scan path on the pattern-line. They usually include an electronic resolver that consists of a sine/cosine generator for producing four sine waves mutually shifted in phase by 90 degrees. These sine and cosine waves are each fed to a sample and hold circuit and a pulse train derived from signals from the photocell momentarily opens the sample and hold circuits at a given phase angle and causes capacitors in the sample and hold circuits to be charged to DC values corresponding to the instantaneous value at the given phase angle of the cosine and sine waves from the generator. These DC voltages are fed to X and Y servo-amplifiers on a machine frame for the tracer to drive a compound slide in a direction corresponding to the selected angle of the sine and cosine waves.
In automatic steering systems the phase of the pulses from the photocell controls the opening of the sampling and hold circuits and such a photocell processing system is illustrated in my co-pending U.S. patent application Ser. No. 108,549 filed Dec. 31, 1979, and entitled An Optical Pattern Tracing System, and corresponding International application PCT/U.S. No. 80/01717, filed Dec. 23, 1980, and reference should be made thereto for a complete description of the scanner mechanism and the manner of processing the photocell signals for this purpose.
Some of these prior optical pattern tracing systems include a pattern approach and catching sub-system that enables an operator to select a manual steering direction for approaching the pattern to be followed. One such sub-system includes a mechanical resistive potentiometer that is connected to receive the sine waves produced by the electronic generator. The potentiometer includes a rotatably mounted slider that engages four annularly arrayed arcuate resistors arranged to produce a sine wave in the slider having a phase proportional to the angle of the slider on the potentiometer. The slider sine wave is fed through a high gain amplifier that produces a square wave pulse train used as the directional pulse train for manual steering. The operator thus selects the appropriate approach direction by setting the angular position of the potentiometer slider. These resistive type manual resolvers are very expensive and add significantly to the cost of the overall tracing system. Moreover, the metal-to-metal contact in these potentiometers creats wear, which after a relatively short time, requires replacement of the entire potentiometer.
After the pattern-line is approached in a predetermined direction by the tracing head, as set by the manual resolver, the control of the steering function must be switched from the manual mode to the automatic mode. A variety of prior line detection circuits produce signals which operate an electronic switch so that the manual directional pulse train is replaced by a similar signal from the photocell circuit.
In these prior tracing systems, the direction that the tracing head follows the pattern after catching is fixed and cannot be preselected by the operator of the machine. When the tracing head or scanner is properly over the line, the photocell provides two signals for each complete scan circle. In most tracing systems it is the leading pulse produced by line crossings in the direction of motion that is the one used for control, although some tracers such as the one disclosed in my co-pending application PCT/U.S. No. 80/01717, filed Dec. 23, 1980, may utilize the trailing pulse for control. It is this leading pulse control that compels prior tracing systems to one direction pattern catching.
To achieve leading pulse control, second pulse blanking circuits have been provided in prior tracers that in response to a first pulse from the photocell, derive a blanking signal that blanks the second pulse so that the sample and hold circuits respond only to the leading pulse. The effect of this circuitry is that line catching can only be unidirectional, either clockwise or counterclockwise, without selectivity. As the scan path approaches the pattern the first intersection of the scan path with the line will provide the first pulse to the blanking circuit and the blanking circuit will thus always blank the second pulse (or vice versa). Since the first pulse is always on one side of the scan path, automatic tracing always proceeds in the same direction.
The inability of prior optical pattern tracing systems to easily preselect the tracing direction represents a severe limitation in the capability of these systems.
A further problem in prior art optical pattern tracing systems that incorporate line catching and locking circuitry is that these systems normally follow the edge of a line-pattern and sometimes use steering corrections or time delay of variable preset magnitude to introduce error signals that compensate for the displacement error caused by the distance between the edge and center of the line.
The second pulse blanking circuit described generally above for blanking the trailing pulse so that the sample and hold circuits respond only to the leading pulse in the direction of movement, also introduces a serious error into the system. An exemplary second pulse blanking system used with rotary scanners incorporates a one-shot flip-flop triggered by the first steering pulse from the photocell and used to mask the second pulse. This one-shot is typically set for a duration of approximately three-quarters of a scan path cycle time. Such an approach works well when pattern lines are well drawn and free from imperfections, but when this is not the case and the first pulse does not appear, the one-shot is triggered by the next available pulse which would be the second pulse resulting in the tracer reversing its direction along the pattern-line.
It is a primary object of the present invention to ameliorate the problems noted above in control circuitry for optical pattern tracing systems of the type having a closed scan path.